In Internet Protocol (IP) networks such as the Internet, a transmission source device that transmits a packet obtains a maximum packet length when transmitting a packet to a destination device. Here, the maximum packet length corresponds to the maximum value (Path Maximum Transfer Unit, PMTU) for the size of a packet that may be transmitted on the shortest route used when a packet is transmitted from the transmission source device to the destination device. The transmission source device transmits to the destination device a packet whose size is set to the PMTU. Moreover, the transmission source device stores the value of the obtained PMTU in association with the destination device so as to be used for the subsequent transmission of a packet.
As a related technique, a device capable of obtaining a PMTU for every route when there are several routing paths and notifying the transmission source device of the packet of the obtained PMTU has been designed.